Microirrigation (also see Online Resources: Irrigation & Water Management)
Microirrigation is an irrigation system that wets only a portion of the orchard root zone. It is recommended that these systems wet approximately 30-60% of the orchard root zone volume.
Components of a Microirrigation System
- pump – size is determined by flow-rate and pressure required
- flowmeter – to determine the amount of water applied
- mainlines and submains – must be sized carefully, best done by a qualified system designer
- drip or microsprinkler lateral lines – length and diameter depends on tube size, cost, and pressure
- valves – for regulating a constant pressure, types: control, air/vacuum relief, and check
- filter(s) – to prevent clogging by removing particulate and organic matter
- injection equipment – for application of chemicals such as fertilizer
- emitters – for delivery of water to crop, types: punch-in, in-line, or microsprinklers, all can be either pressure compensating or not. Emitter spacing and discharge rate required depend primarily on tree spacing and water needs.
- Surface Drip: Surface drip irrigation, along with microsprinkler irrigation, is the most commonly-used microirrigation systems in tree crops. The drip emitters can either be “punched-in” to the drip tubing or are formed integrally in the drip hose (“in-line” emitters). Both single and double drip lines per tree row systems can be used. Double drip lines are frequently chosen to achieve a greater wetted area.
- Subsurface Drip: Orchard subsurface drip irrigation (SDI) systems are most often integral “in-line” drippers in hard drip tubing. Both single-line and double-line subsurface drip systems can be used. Subsurface drip systems’ installation depth is usually 10” to 24”. Advantages of subsurface drip systems include reduced weed growth, reduced irrigation system damage during orchard maintenance, and the ability to irrigate almost anytime. The disadvantages of subsurface drip are the difficulty in detecting clogging problems and the hazard of root intrusion into the emitters by expanding tree roots.
- Microsprinklers: Advantages of microsprinklers compared to drip systems include a larger wetted area, often a higher application rate, less susceptibility to particulate and chemical clogging, and easier visual inspection for problems. Disadvantages of microsprinklers include increased weed growth, increased water use due to higher evaporation rates, and clogging due to insects entering or laying eggs in the emitters.
Deterimining Irrigation Frequency and Duration
When designing an irrigation system, it is better to over-estimate the application rate and later decrease emitter flow rates if needed. It is also helpful to monitor soil moisture with an auger both before and after irrigation to observe the depth of water infiltration. comparing what has worked on other orchards with similar soil conditions can also provide guidance.
- Step 1: Estimate tree water use: Ideally this is based on knowledge of tree water use, which is measured as ET – the total amount of water evaporated and transpired from leaves, measured in inches/day. Since irrigation systems typically measure water output in gallons per hr, we use a formula to calculate Tree Water Use in gallons per day: tree spacing x ET x 0.623 . Table 1 provides Tree Water Use on a 'per tree' basis for various tree spacings (tree spacing being an indicator of canopy size). Note that the values are rounded up to the nearest whole number.
Table 1. Tree Water Use in gallons per day1
|Evapo-transpiration rate (ET) in inches/day, indicating tree water use|
1 The Tree Water Use = tree spacing x ET x 0.623
2 Tree Spacing (ft2) = row spacing (ft) x tree spacing within row (ft)
- Step 2: Set the number of emission devices per tree and the discharge rate per device in gallons per hour per device:
Application Rate (gal/hr) = Number of Emitters x discharge rate per device (gal/hr/emitter)
Drip Emitter example:
4 gal/hr/tree = 4 emitters/tree x 1 gal/hr/emitter
12gal/hr/tree = 1 sprinkler/tree x 12 gal/hr/sprinkler
- Step 3: Set irrigation system operation time in hours per day. Use Tree Water Use (Step 1) and Application Rate (Step 2) for this calculation. Table 2 is a chard of operation times for various application rates and tree water use values.
Hours of Operation/day = Tree Water Use (gal/day) / Application Rate (gal/hr)
Drip Emitter Example:
12.5 hrs/day = 50 (gal/day) / 4 (gal/hr/tree)
4.16 hrs/day = 50 (gal/day) / 12 gal/hr
Table 2. Hours of operation for various Application Rates & Tree Water Use
|Application Rate (gals/hr)|
|Tree Water Use
The above calculations may be adjusted by other considerations: to maintain aeration in the root zone, the system should not run continually, despite hot, dry seasonal conditions.
- A prudent design suggests that a drip system operate about 1/2 to 2/3 of the time during peak ET periods (not continually).
- For a microsprinkler system, 3 to 7 days between irrigations is common during peak ET periods. Microsprinkler irrigation periods should be long enough for deep penetration into the root zone to occur (24 - 48 hours).
- In all cases, soil moisture monitoring using an auger or similar device in useful before and after irrigation to determine where water is taken up by the tre and how deep water has penetrated.
- Salinity and the existence of shallow groundwater in the orchard are considerations to factor into irrigation decisions.